NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOX catalysts, selective catalytic reduction (SCR) catalysts, and lean NOx traps (LNTs).
Lean-burn NOX catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOX in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NOx reduction, introducing a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are devices that adsorb NOx under lean conditions and reduce and release the adsorbed NOx under rich conditions. An LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkali or alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalyst activates reactions by which hydrocarbon reductants are converted to more active species, the water-gas shift reaction, which produces more active hydrogen from less active CO, and reactions by which adsorbed NOX is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
Regeneration to remove accumulated NOx may be referred to as denitration in order to distinguish desulfation, which is carried out much less frequently. The reducing environment for denitration can be created in several ways. One approach uses the engine to create a rich exhaust-reductant mixture. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. A reducing environment can also be created by injecting a reductant into lean exhaust downstream from the engine. In either case, a portion of the reductant is generally expended to consume excess oxygen in the exhaust. To lessen the amount of excess oxygen and reduce the amount of reductant expended consuming excess oxygen, the engine may be throttled, although such throttling may have an adverse effect on the performance of some engines.
Reductant can consume excess oxygen by either combustion or reforming reactions. Typically, the reactions take place upstream of the LNT over an oxidation catalyst or in a fuel reformer. The reductant can also be oxidized directly in the LNT, but this tends to result in faster thermal aging. U.S. Pat. Pub. No. 2003/0101713 describes an exhaust system with a fuel reformer placed in an exhaust line upstream from an LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate.
Regardless of how the reducing environment is created, it is important to control the frequency with which reducing conditions are created. If the frequency of regeneration is too low, the LNT will fail to perform its function effectively. If the frequency of regeneration is too high, the fuel penalty becomes excessive. In any event, the fuel penalty for regenerating an LNT is a significant factor contributing to the operating cost of a vehicle using an LNT and it is desirable to keep that fuel penalty as low as possible while still meeting emission control objectives.
Numerous methods for scheduling LNT denitrations have been proposed. The simplest method is periodic regeneration: regeneration is conducted after a fixed period of lean operation. This method is generally impractical in that NOx accumulation rates vary widely over the course of vehicle operation. Using periodic regeneration, either the fuel penalty will be unacceptably high or the emissions control will be unacceptably low.
A prevalent method for scheduling LNT denitration is to schedule based on LNT loading. LNT loading can be characterized in terms of amount of NOx accumulated, remaining NOx storage capacity, percent saturation, or another parameter of this type. Numerous methods for estimating NOx loading and/or remaining NOx storage capacity have been proposed. These methods generally involve integrating an estimate of the NOx storage rate and comparing the result to an estimated NOx storage capacity.
NOx storage rates can be estimated from differences between NOx flow rates out of the engine and NOx flow rates out of the LNT or by multiplying NOx flow rates out of the engine by estimates of LNT storage efficiency. Engine out NOx flow rates can be estimated exclusively from engine operating maps or using a NOx sensor in the exhaust upstream from the LNT. NOx flow rates out of the LNT, when used, are generally estimated using NOx concentration sensors.
Regenerating based on LNT loading is better than regenerating periodically, but is still inaccurate in the sense of resulting in overly frequent or infrequent regenerations. Aside from any inaccuracies in measuring NOx storage rates, it is difficult to accurately determine NOx storage capacity. NOx storage capacity varies over time due to factors including, without limitation, sulfur poisoning, catalyst aging, and catalyst temperature. The degree of saturation at which LNT efficiency becomes unacceptably low is also variable being a function of these and other factors.
Another limitation to regenerating based on NOx loading is that it does not take into account the performance of the entire exhaust treatment system. It is known that an LNT can produce ammonia during denitration and from this knowledge it has been proposed to combine an LNT and an ammonia SCR catalyst into one system. Ammonia produced by the LNT during regeneration is captured by the SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone LNT with little or no increase in fuel penalty or precious metal usage. Regeneration of an LNT in a hybrid system based on LNT loading only may be premature due to performance of the SCR catalyst in addition to the other factors mentioned above.
An alternative approach is to schedule LNT regeneration based on current performance of the aftertreatment system as determined from NOx concentration measurements taken downstream from the LNT. These measurements can be used on a standalone basis, regenerating when the downstream concentration exceeds a critical value, or in combination of with estimates of NOx concentration upstream from the LNT, whereby the LNT performance efficiency can be determined and used as a criteria. The performance of the LNT can be determined individually, or the performance of the LNT in combination with another device, such as an SCR reactor, can be measured.
In spite of advances, there continues to be a long felt need for an affordable and reliable diesel exhaust aftertreatment system that is durable, has a manageable operating cost (including fuel penalty), and reduces NOx emissions to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.